A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
The reticle (also called mask) may be reflective or transmissive. A reflective reticle reflects a patterned version of the projection beam and the reflected beam is directed to the substrate. A transmissive reticle transmits a patterned version of the projection beam and the transmitted beam is directed to the substrate. The reticle often contains a glass plate, on one side of which the pattern is provided, e.g. in the form of a patterned Chromium layer. To protect the patterned surface of the reticle, a pellicle (which is made of a transparent material in the form of a thin foil or a thin glass plate, for example) is provided, which covers the patterned surface of the reticle. The pellicle is attached to the reticle using a frame that connects to the edges of the pellicle, leaving a pellicle space free. This frame will be called the pellicle frame.
U.S. Pat. No. 6,507,390 discloses a pellicle-reticle-frame assembly in which the pellicle frame is porous, thereby permitting an inert purge gas to flow into and out of the pellicle space. Alternatively, holes may be provided in the pellicle frame to permit the flow of purge gas. Purging of spaces surrounding optical equipment is a standard procedure. During conventional purging, a steady flow of an inert gas is maintained to drive out unwanted gases.
U.S. Pat. No. 6,507,390 describes how the pellicle-reticle-frame assembly can be purged in preparation of use. This is often done each time before the reticle is loaded into the reticle stage of the photolithographic apparatus, shortly before the reticle will be used during exposure of the substrate. The pellicle-reticle-frame assembly is placed in a box in which purge gas flow is realized through the pellicle space in parallel with the surface of the reticle and the pellicle. A purge gas supply and a vacuum source are mounted near the edges of the pellicle frame on mutually opposite sides of the pellicle space. Purge gas flows from one edge to the other between the pellicle and the reticle.
The pellicle is a flimsy structure that is often prone to damage. Therefore, direct (accidental) contact between the pellicle and the outlets of the purge gas supply and the vacuum source should be avoided. Nevertheless, these outlets should be located as close as possible to the edges of the pellicle frame to minimize loss of gas that does not flow through the pellicle space. This often requires the overhead of accurate positioning equipment. When the pellicle size and shape are different for different reticles, careful adaptation should be used.
Another problem with the purge gas flow approach is that contaminants, like water, that are adsorbed to pores of the pellicle frame on the side of the purge gas supply flow into the pellicle space and are often removed through the pellicle frame on the side of the vacuum source, where they may again be adsorbed to the pellicle frame. This may extend the time that a purge flow is needed. Particles blown into the pellicle space from the pellicle frame may get attached to the reticle, often making the reticle useless.
A further problem with the purge gas flow from one side of the pellicle to the other is that different dynamic forces may be generated on different sides of the attachment to the pellicle frame, which may lead to deformation and a spatial relation between the pellicle and the reticle.